1. Field of the Invention
The present invention relates generally to external defibrillators, and particularly to charging systems and algorithms for external defibrillators.
2. Technical Background
Sudden Cardiac Arrest (SCA) is a condition in which the heart exhibits a life-threatening abnormal rhythm, or arrhythmia. The most common arrhythmia is Ventricular Fibrillation (VF). When in VF, the heart's rhythm is so chaotic that the heart merely quivers, and is unable to pump blood to the body and brain. This chaotic rhythm is generally referred to as fibrillation. A victim in SCA first loses his or her pulse, then consciousness, and finally the ability to breath. These events happen in a matter of seconds.
An effective treatment for SCA is to deliver an electrical shock using a device called an external defibrillator to defibrillate the heart. Voltage stored by the external defibrillator is applied across the patient's heart by means of electrodes or paddles place on the victim's body, such as the victim's chest, resulting in an electrical current flow through the heart. The brief pulse of electrical current is provided to halt the fibrillation, giving the heart a chance to start beating with a normal rhythm. This delivering of the electrical shock, which is intended to return the heart to normal rhythm, is called defibrillation.
Survival rates for SCA are the highest when defibrillation is conducted within the first few minutes after onset of an arrhythmia, and the person has the best chance of survival if the defibrillation shock is given within the first three minutes of the person's collapse. One study has shown that the chances of resuscitating an individual suffering SCA are reduced by about 7% to about 10% with each minute that lapses between onset of SCA and application of the defibrillation shock. Therefore, rate of survival for SCA victims average less than 2% when defibrillation is delayed ten minutes or more.
There are several types of external defibrillators, including manual defibrillators, which are generally used by medical personnel, and automated external defibrillators, commonly known by the acronym AED, which are designed for use by laypersons. Typically an AED is a small, portable device that analyzes the heart's rhythm and if the analysis determines that a defibrillating shock is advisable, either prompts the user to deliver a defibrillation shock or delivers a defibrillation shock without user interaction. Once a typical AED is activated, it can guide the user through each step of the defibrillation process by providing instructions in the form of aural or visual prompts.
Commercially available AED's analyze the patient's heart rhythm and charge the energy storage capacitor before a defibrillating shock can be delivered to the patient. This is done using a decision-making algorithm commonly referred to as a shock advisory algorithm. Current commercially available defibrillators analyze the ECG first to determine if the patient's heart is in a condition where delivery of a defibrillating shock is advisable, and after this analysis is done, charge if a shock is recommended. Typically, a shock advisory algorithm may take from about five to about 8 or more seconds to analyze, and the capacitor in a typical AED typically takes about 8 to about 10 or more seconds to charge up in preparation for delivery of a defibrillating shock. Time delays such as this have a negative impact on patient survival and should be minimized; the amount of time required for a defibrillator to get ready to deliver should be kept as short as possible.
Several approaches to reducing the time to shock have been proposed. One approach is to design device hardware so as to minimize the charging time. Unfortunately, this can raise manufacturing costs. Another approach is to commence charge of the defibrillator upon turning the device on. If the shock advisory analysis indicates that the patient requires a shock, then the device is ready to deliver the energy. But, if the analysis indicates that no shock is required, then the device will internally dump the stored charge. This results in a waste of energy that can result in increase cost of operation due to increased need for battery replacement. There also is concern about the potential for accidental delivery of energy since the charge will become available whenever the device is activated, whether needed for therapy or not.